The present application is a U.S. National Stage of International Application No. PCT/AT99/00034 filed Feb. 16, 1999 which claims priority under 35 U.S.C. xc2xa7 119 of Austrian Patent Application No. 247/98, filed on Feb. 11, 1998.
1. Field of the Invention
The present invention relates to a process for precipitating solid compounds from the liquid zinc or liquid zinc-based alloys of a metal bath thereof.
The invention further relates to a device for precipitating solid compounds from a molten mass of liquid zinc or liquid zinc-based alloys, in particular from a metal bath thereof, for example, a galvanizing bath.
Finally, the invention relates to the use of the process and the use of the device for precipitating compounds and oxides, particularly intermetallic iron compounds from galvanizing baths, in particular from continuous sheet galvanizing systems.
2. Discussion of Background Information
Zinc is a metal with a comparatively low melting point of 419.6xc2x0 C. and has a standard electrode potential of xe2x88x920.762 V, and is therefore baser than iron with xe2x88x920.447 V. In addition, zinc is relatively stable towards oxygen, because a zinc oxide layer formed on a zinc surface protects the zinc from the attack by oxygen. Thus, if a steel part is coated with a zinc layer, the zinc acts as an anode, i.e., protects the iron from oxidation, with the formed zinc oxide preserving the zinc layer from further oxidation. Because long-term rust protection of iron and steel can be achieved by zinc and zinc has a melting point that is lower by more than 1100xc2x0 C., galvanizing parts in a liquid zinc bath is one of the most important protective measures against atmospheric and similar corrosion of steel components.
Galvanizing the steel parts takes mostly place by dipping them into a bath with liquid zinc or a zinc-based alloy to form a coating. Sheet metal is passed through a zinc bath, for example, which allows to achieve a thin, uniform and perfectly smooth surface coating.
Good adhesion of the zinc layer is ensured due to the solubility of the zinc in the alpha (xcex1)-mixed crystal of the iron of up to approximately 7.3 at. % at usual bath temperatures. On the other hand, iron is only slightly soluble in zinc at its melting temperature, with the three-phase equilibrium at 419.35xc2x0 C. having an eutectic composition: zinc and 0.021 at. % or 0.018% by weight of iron. At higher temperatures of 530xc2x0 C., for example, the solubility of iron in liquid zinc is 0.3 at. % or 0.25% by weight.
If iron is introduced into a galvanizing bath due to iron dust or the like adhering to the part as well as by system parts, then iron-zinc mixed crystals are formed, for example FeZn3, FeZn7, Fe3Zn10, Fe5Zn21. These FeZn mixed crystals may be heavier than pure zinc and enrich themselves at the bottom of quiescent liquid galvanizing baths as hard zinc or as so-called xe2x80x9cdrossxe2x80x9d or xe2x80x9cbottom dross.xe2x80x9d
In order to improve the quality of the galvanizing or the zinc layer formation, in particular on metal sheet metal which passes through the zinc bath at a high speed, aluminum can be added at an order of magnitude of 0.1 to 0.2% by weight to form an alloy with the zinc bath. In the given case, the xe2x80x9cbottom drossxe2x80x9d interacts, in terms of reaction kinetics, with the zinc molten mass that due to subsequent alloying contains aluminum, and iron-aluminum mixed crystals with a configuration of Fe2Al(5xe2x88x92X)Znx and a specific weight of significantly less than 6xc3x97103 kg/m3 are formed.
The mixed crystals, in particular the compounds Fe2Al(5xe2x88x92X)Znx, form a pulp in the galvanizing bath, and with a continuation of the galvanizing treatments and further introduction of iron into the bath the particles become larger, reach a diameter of over 30 xcexcm and agglomerates, so-called clusters, are generated. These coarse compound particles in the galvanizing bath, which may be in the form of lumps, can cause surface defects or can adversely affect the surface quality of the coating, especially in the galvanization of smooth sheet metal bands, such as those for the automobile industry, for example.
In order to purify galvanizing baths contaminated with intermetallic compounds, it is necessary to render the bath quiescent with respect to bath currents, if possible, whereafter the layer enriched on the surface with the compounds is skimmed off. Performing this kind of purification in sedimentation basins has already been proposed.
The previously known purification processes all have the disadvantages of low efficiency, high expenditure, reduced economic efficiency, and production safety, as well as productivity of the system.
The object of the invention is to eliminate the deficiencies of previous precipitation methods and to disclose a process by means of which even with large throughput quantities an insignificantly low content of solid compounds can be maintained in galvanizing baths.
An additional object of the invention is to provide a device for the optionally continuous precipitation of solid compounds from galvanizing and zinc baths, which device does not impede production.
The object is attained in a process of the type recited at the outset by subjecting partial amounts of the liquid metal phase containing the compound(s) and being above the melting temperature of zinc to an acceleration higher than the acceleration due to gravity and causing an at least partial demixing into fractions containing heavier and/or lighter components, whereafter the molten mass depleted of solid compounds is returned to the metal bath or made available in purified form for such a bath, and the part of the molten mass enriched with the desired compounds is discharged and/or supplied to a further separation process.
The additional object of the invention is attained with a device according to the species in that a hollow rotary body that can be driven about an axis and has conveyor means, such as conveyor impellers or pump impellers, projecting into the cavity in the feed or lower portion is introduced at least partially into the molten mass, which hollow rotary body is provided in its discharge or upper portion with at least one discharge opening for the depleted molten mass, which opening is eccentrically arranged in the wall of said body and, subsequent thereto with respect to the discharge side, with at least one further centrally and/or eccentrically arranged discharge opening for the liquid metal enriched with compounds wherein at least one of the upper molten mass discharge openings in the hollow rotary body opens into a discharge area of a housing which at least partially surrounds the hollow rotary body, which housing area has at least one discharge channel for the enriched molten mass and, optionally, a further one for the depleted molten mass designed to return to the metal bath.
The advantages of purifying a galvanizing bath of intermetallic compounds achieved in such a way can essentially be seen in that with a high acceleration of the metal contaminated with compounds of different densities a high degree of demixing can be achieved and utilized. A rapid and highly effective separation or accumulation of solid suspended particles, in particular Fe2Al(5xe2x88x92X)Znx, in the center of a rotating molten mass, was surprising for the person skilled in the art because on the one hand the difference in the specific weights is to be considered as rather small and, on the other hand, the low particle size and especially the phase boundary tensions between the compound and the liquid metal counteract demixing. The further advantage according to the invention is that, as was found, larger intermetallic particles and particularly compound agglomerates, which can cause an especially large reduction in quality of the coating, can be concentrated and discharged fully and in a highly effective manner by high acceleration.
If in so doing the molten mass containing the compounds is subjected to a centrifugal acceleration of at least 1.2 times, preferably of at least 2.1 times, particularly of more than 10.1 times the acceleration due to gravity, an efficient purification effect can be achieved.
To achieve high productivity, it is advantageous for the precipitation of compounds to be conducted essentially continuously with continuous removal of partial amounts of contaminated molten mass from a zinc (alloy) bath of an iron galvanizing device, in particular of a steel sheet galvanizing system, with return of the depleted liquid metal to the bath.
The purification effect of the process can be increased further if before or during the demixing treatment by applying an increased acceleration on the metal phase containing the compounds said mixture is adjusted or heated or cooled to a treatment temperature to between 420xc2x0 C. and 480xc2x0 C., preferably to between 429xc2x0 C. and 450xc2x0 C. This results in establishing a low solubility of the compounds in liquid metal and thus a precipitation or growth of nuclei or particles of the intermetallic phase.
The advantages of a device according to the invention essentially consist in that by the conveyor means projecting into the cavity of the hollow rotary body through the feed side a throughput and an increase in pressure of the rotating molten mass therein can be achieved . Both the amount fed through and the pressure in the liquid metal are, as was found, important for precipitation kinetics and the concentration behavior of the solid compounds in the liquid metal when centrifugal acceleration is applied. Both the flow rate through the hollow rotary body as well as the pressure built up therein can be adjusted synergistically with respect to optimal precipitation criteria of intermetallic particles by the design of the conveyor means and by the size of the discharge openings.
If the device is put out of operation by turning off the drive and is lifted, the entire interior space runs dry so that it is available for re-use without further cleaning work.
The rotation of the contaminated metal molten mass induced by the conveyor means can be supported effectively, whereby the hollow rotary body can also be reduced in terms of its length, if the interior wall of the hollow rotary body comprises carrier or guide means promoting the rotational movement of the molten mass therein.
In this connection, a separate molten mass discharge is improved if the hollow rotary body has centrifugal sealing elements or similar sealing elements cooperating with the housing parts.
It is advantageous for the input speed of the rotating shaft of the hollow rotary body to be adjustable in order to adjust the optimal flow rate through the hollow rotary body and the pressure inside the hollow rotary body, which are important particularly for varying temperatures of the molten mass.
It has proven to be favorable for both reducing the heat removal and for draining the device of liquid metal if at least parts of the housing have heat insulation.
In one aspect, the present invention relates to a process for removing solid compounds from a molten metal bath, the molten metal selected from liquid zinc and liquid, zinc-based alloys. The process comprises subjecting the molten metal containing the solid compounds to an acceleration higher than the acceleration due to gravity, whereby at least partial demixing of the molten metal into fractions comprising at least one of heavier and lighter components is effected. A fraction enriched with the solid compounds is separated from a fraction depleted of the solid compounds and the molten metal depleted of the solid compounds is returned to the metal bath or used in a new bath. The molten metal enriched with the solid compounds is discharged and/or supplied to a further separation process.
In one embodiment the solid compounds comprise intermetallic compounds, metal oxides or both. The intermetallic compounds may comprise iron and zinc, or iron, zinc and aluminum.
According to another embodiment of the above process the acceleration higher than the acceleration due to gravity is centrifugal acceleration. This centrifugal acceleration may be at least 1.2 times, preferably at least 2.1 times, and more preferably more than 10.1 times the acceleration due to gravity.
In a further embodiment of the instant process the metal bath is a galvanizing bath; for example, the metal bath may be part of an iron galvanizing device, e.g., a steel sheet galvanizing system.
Another aspect of the present invention is a process for removing solid compounds from a molten metal derived from a metal bath and selected from liquid zinc and liquid, zinc-based alloys, wherein the molten metal containing the solid compounds is subjected to an acceleration higher than the acceleration due to gravity, thereby effecting at least partial demixing of the molten metal into a fraction enriched with the solid compounds and a fraction depleted of the solid compounds. The density of the fraction enriched with the solid compounds is different from the density of the fraction depleted of the solid compounds. The fraction enriched with the solid compounds is separated from the fraction depleted of the solid compounds, whereby molten metal depleted of the solid compounds for further use in a metal bath is obtained.
In a further aspect the present invention relates to a device for precipitating solid compounds from a molten mass composed of liquid zinc or liquid, zinc-based alloys. The device comprises a hollow rotary body, rotatable about an axis thereof, that includes a wall, a lower feed portion and an upper discharge portion. The device further comprises a housing which at least partially surrounds the hollow rotary body and has a discharge area that includes at least one discharge channel for molten mass depleted of solid compounds. Also included in the device is a conveyor that is arranged at least partially into the molten mass and projects into a cavity in the lower feed portion. The device further includes at least one first discharge opening for molten mass depleted of solid compounds, located in the upper discharge portion and eccentrically arranged in a wall of the hollow rotary body, and at least one second discharge opening for molten mass enriched with solid compounds, the second discharge opening being arranged centrally and/or eccentrically in the upper discharge portion of the hollow rotary body and after the first discharge opening with respect to the direction of discharge. At least one of the first and second discharge openings opens into the discharge area of the housing.
In one embodiment the conveyor comprises conveyor impellers or pump impellers.
According to another embodiment the interior wall of the hollow rotary body is designed to promote a rotational movement of the molten metal inside said hollow rotary body. For example, the interior wall may have carrier or guide elements.
In further embodiments of the device the hollow rotary body additionally has centrifugal sealing elements which cooperate with the housing, and at least parts of the housing have heat insulation.
According to a still further embodiment the discharge area of the housing has discharge channels for both the first and second discharge openings, while in another embodiment the discharge channel for the first discharge opening is designed to return the molten mass depleted of solid compounds back to original molten mass.
Another embodiment of the device includes a hollow rotary body that has a rotating shaft. The input speed of the rotating shaft may be adjustable.
In a further embodiment the discharge area of the housing also includes at least one discharge channel adapted for discharging the molten mass enriched with solid compound.